Detailed knowledge of the groundwater flow regimen and interaction between groundwater and surface waters is a pre-condition for an understanding of geochemical processes in the subsurface. Groundwater flow velocities and directions, for example, are essential parameters for delineating trends in As concentrations and to quantify reaction rates. The hydrogeology core laboratory will provide the tools and expertise for collection and analysis of hydrogeologic data for projects numbers 5,6, and 7. The following types of data will be obtained and interpreted: hydraulic head, grain size and porosity, geophysical data (resistivity, ground penetrating radar and land seismic) and tracer data (SF6 3H/3HE, 14C, 13C, 18O, D). The equipment to be proceed and operated by the facility will include multilevel wells, in situ groundwater monitoring devices, sample preparation systems for isotopic analyses, a multi- channel seismic system, a multi electrode resistivity meter, a frequency em conductivity system, ground penetrating radar, differential GPS systems, gas chromotographs, and noble gas spectrometers. The research core will also provide three-dimensional modeling capabilities for groundwater flow simulations. The geophysical techniques including frequency em conductivity, ground-penetrating radar, resistivity and high resolution reflection seismics will be used to develop the stratigraphy in projects 5 and 6. Analyses of tritium, noble gas isotopes, SF6, carbon isotopes and stable water isotopes performed by gas chromatography and mass spectrometry will be used to derive groundwater residence times for a range of time scales (months to 10,000 of years). The combination of stratigraphic, hydraulic and tracer data will be used to build and constrain groundwater flow and transport models for projects 5, 6, and 7.